Investigations into organic photo-semiconductors have been made with a view to utilizing them in electrophotography. The photoconductivity of polyvinylcarbazole and the use of polyvinylcarbazole in electrophotography are known, e.g., as described in German Patent Publication No. 1,068,115 and U.S. Pat. No. 3,037,861. The above-described published patent specification also describes that the photosensitivity of polyvinylcarbazole can be enhanced by addition of a small amount of dye.
Electrophotographic recording materials having photoconductive films formed from photoconductive compositions containing 0.49 to 1.23 mole of 2,4,7-trinitro-9-fluorenone per mole of constitutional repeating unit of a polymerized heterocyclic vinyl compound, such as polyvinylcarbazole, are known, e.g., as described in German Patent Publication No. 1,572,347 and U.S. Pat. No. 3,484,237. In addition, the electrophotographic recording material described in German Patent Application (OLS) No. 1,797,561 includes a photoconductive film formed from a photoconductive composition containing equiparts (by weight) of 2,4,7-trinitro-9-fluorenone and polyvinylcarbazole. R. M. Schaffert reports in IBM Journal of Research and Development, Volume 15, No. 1, pages 75-89 (1971) that the above-described recording materials possess excellent various properties which are attributable to the charge transfer complexes constituting their respective photoconductive films. Furthermore, R. M. Schaffert, supra, at page 76 describes that many substances reported to be able to act as a sensitizer for polyvinylcarbazole have poor compatibility with polyvinylcarbazole, and they often affect adversely the photosensitivity of polyvinylcarbazole when used in high concentrations. Furthermore, R. M. Schaffert, supra, at page 77 describes that none of these many substances is as useful as 2,4,7-trinitro-9-fluorenone, and that substances possessing effectiveness equal to or greater than that of 2,4,7-trinitro-9-fluorenone have never been found. However, the trinitrofluorenone is expensive and has extremely high physiological activity. Therefore, it has been an important subject in this art to discover compounds having sensitizing abilities equal to that of the trinitrofluorenone without having the disadvantages of the trinitrofluorenone.